The invention pertains to close-outs for sandwich structures which have either rounded or angled corners, the corners being formed by unfolding and expanding.
During recent years, sandwich structures have attained widespread use in the aircraft industry in wings, wall panels, beam webs, propeller and engine blades, stablizers and stabilators, and control surfaces.
Superplastic forming properties are exhibited by only a small number of materials and alloys, and the process involves the capability of a material to develop unusually high tensile elongations and plastic deformation at high temperatures with a reduced tendency towards thinning or necking. The workpiece is heated until it becomes superplastic, at which time a pressure differential is applied causing the workpiece to stretch into a desired shape. Aluminum alloys and titanium alloys exhibit good superplastic characteristics.
Diffusion bonding is often the preferred method of joining these superplastically formed workpieces. The method is a metallurgical joining of surfaces by applying heat and pressure for a time sufficient to cause commingling of the molecules at the joint interface. The basic requirement for diffusion bonding is to bring the clean mating surfaces close enough together to allow the inter-molecular attractive forces to become effective. A stop-off material is used to coat those portions of the surfaces where diffusion bonding is not needed. Diffusion bonding is accomplished entirely in the solid state.
However, superplastic forming, even when used with diffusion bonding, has several distinct disadvantages: (1) only the few materials which exhibit superplasticity may be used, (2) the structures must be raised to the high superplastic forming temperatures and pressures, (3) the considerable stretching produces structural distortions, and (4) the core thickness is limited since excessive stretching weakens the sandwich structure.
A novel forming process known as "accordion expansion" overcomes these disadvantages. The process is disclosed in U.S. application Ser. No. 466,987, entitled "Accordion Expansion Process" by Leonardo Israeli, which is incorporated into this specification by reference. This method makes sandwich structures by using two face sheets and a plurality of core sheets that unfold during the forming process. Although accordion expansion involves some stretching of the core sheets, the stretching is much less than what is involved in superplastic forming. Although accordion expansion occurs at elevated temperatures, such temperatures are generally lower than superplastic forming temperatures. A wide variety of materials (many of which cannot be used in superplastic forming) may be used in accordion expansion including aluminum, titanium, copper, and their respective alloys, and steel.
However, in order to fabricate completed sandwich structures by accordion expansion, it is also necessary to form the close-outs. Generally, sandwich structure must be substantially sealed and closed out. In addition to protecting the formed core, this close-out structure adds considerable strength to the sandwich structure. Although the close-out structure can be independently formed and assembled, it is preferred to form it in a one-step operation with the core sandwich. Furthermore, if the close-out structure is made of material similar to the core material in this one-step operation, the same temperature and pressure ranges can be utilized.